Apparatus of this kind is described in U.S. Pat. No. 4,591,487, by the inventor hereof, to which German Pat. No. 30 30 541 corresponds. This apparatus has several shafts arranged vertically on a theoretical ring, with parallel axes and driven in the same direction. Each shaft carries a number of disk-shaped processing elements arranged axially in a row and in parallel planes, meshing with processing elements on adjacent shafts. The shafts and processing elements define at least one cavity therebetween, in which the processing elements are arranged in such a way that their circumferential surfaces are exposed. The shafts have their ends rotatably mounted inside a housing, which also also has arrangements for the intake and discharge of the material to be processed. An outer housing closely surrounds the envelope of the processing elements. The housing wall surrounding the disk-shaped processing elements is formed with tub-shaped recesses of the housing wall, which are arranged in a rosette-shaped housing liner, in whose tub-shaped recesses the processing elements of each shaft mesh with very narrow play. The cavity enclosed by the shafts can be connected to a negative pressure source, so that it functions as degassing chamber for the material which is continuously transported toward the cavity by the processing elements and which surrounds the cavity wall in the form of a drum-like layer which has the characteristics of a "thick film".
In the area of the narrow gap between the outer circumferential surfaces of the processing elements and the adjacent wall of the tub-shaped recesses surrounding the processing elements on the side opposite the cavity, the material being processed is locally subjected to a very high degree of shearing stress which can damage the material being processed. These locally occurring stress peaks are largely uncontrollable because they are influenced by such factors as misalignment of the housing, oscillations of the shafts, non-homogeneities of the material for processing, etc. For example, in the processing of polymers, local temperature peaks occur in the areas mentioned; these are not directly measurable because they occur in narrowly defined places surrounded by undamaged material, but they can be detected because the processed polymers are subject to a loss of strength that can only be explained by the presence of thermally damaged polymer material.
A screw-type machine, such as the one described in German Pat. No. DE-OS 28 02 125, may be used to incorporate reinforcement fibers into polymers in such a way that the fibers and the polymeric mixture are fed into the apparatus in advance of a plasticizing zone. Upon melting of the polymer, it was found that a large portion of the fibers had been ground into the pure filler or reduced to fiber fragments upon transport through the narrow gaps between the housing wall and the processing elements. This reduces the stability of the material considerably and is a great disadvantage in many areas of application.
The same applies in the processing of thermally very sensitive materials, for example in the production of chocolate, where sugar and fats have to be blended and very finely dispersed. Here the material has to be processed on the one hand with a very high degree of shearing force and, on the other hand, the fats must not turn rancid. But in spite of cooling of the kneader shafts, local overheating can hardly be prevented since for example the processing elements opposite the housing heat and are subject to a certain amount of local wear, together with the housing whose circumferential surface interacts with them.
In any apparatus whose processing elements are enclosed by a housing that surrounds them leaving narrow play, the gap between the processing elements and the housing is not exactly defined. Such apparatus therefore is generally unsuited for processes in which biologically active materials containing microorganisms are handled. Such microorganisms can reach sizes of up to 0.3 mm; they are very susceptible to pressure and crushing.